This study is designed to examine the relationship between access to and acquisition of sterile syringe and drug injection-mediated HIV risk among injection drug users (IDUs). Specifically, the study focuses on three phases in the drug injection process 1) acquiring syringes; 2) using syringes (and other injection equipment) 3) discard of syringes. The study is driven by the primary hypothesis that injection equipment-mediated health risk is a local phenomenon; it is determined by a combination of factors (e.g., presence of syringe exchange program, presence a pharmacy that sells over the counter syringes) that determine the availability of sterile syringes, the accessibility of available sterile syringes (e.g., impact of police pressure, cost of syringes, and transportation on facilitating or diminishing access), and local injection and discard patterns ( e.g., impact of individual characteristics, knowledge and behaviors employed by IDUs during and after injecting). To assess the range of acquisition, use and discard patterns and to determine the individual, neighborhood, and city correlates of safer and riskier acquisition, use and discard behaviors, this study will collect data in three cities, and in eight bounded neighborhoods within each city (for a total of 24 neighborhood sites). The three cities, New Haven, CT, Hartford, CT, and Springfield, MA, provide a natural laboratory that allows the control of important features (e.g., the three cities are of similar size, share regional characteristics as Northeastern cities, have large numbers of injection drug user, have populations of IDUs at high risk for HIV infection, have gang controlled drug distributing systems, share similar patterns for injection drugs of choice, and have similar ethnic compositions) while allowing comparison for contrasting features (e.g., nature of availability of sterile syringes form SEPs and pharmacies, police pressure, transportation, etc). Similarly, within the target cities, neighborhoods will be selected to control for the presence, absence or combination of key features (especially the location of SEP sites and pharmacies). The study will combine biomedical, epidemiological and ethnographic methods to: 1) test syringes acquired and discarded by IDUs for human DNA (to demonstrate that a syringe has been used), antibodies for HIV-1 and HCV and insulin; 2) assess self-reported acquisitions, use , and discard patterns; and 3) observe patterns of drug injection equipment- mediated risk behaviors. Further, the study will test a set of corollary hypotheses concerning risk relationships between syringe acquisition, use and discard by making comparisons across cities and across neighborhoods in term of features of interest ( e.g., high or low availability of sterile syringes) and actual risk factors (e.g., frequency of infected syringes in circulation).